As the title suggests, I need advice on how to fill some voids that have happened due to water runoff under my air conditioner slab.

I've seen YouTube videos using Secure Set foam spray. Seems easy enough, but I only see one size of the product at nearly 400.00 that is used for large patios or driveways. I don't need anywhere near as much.

I also can't drill thru the concrete because of the unit on top, but even if l could, I still need some sort of product in a smaller quantity.

I've thought of using the spray foam used to fill cracks and openings in a wall, but not sure if it's strong enough. I don't need to lift the slab, just support it from underneath where the dirt has washed away.

Thansk for any advice.

I'd spray foam it with the longest straw you can find in order to reach halfway under the slab while working your way out. I recall the white foam curing harder than the yellow for some reason but maybe it was brand specific.

My engineer would want to force/inject a concrete slurry up into those voids and create a sloped perimeter with it BUT that would be structural and most would agree overkill.

1. The "Pro" line of Great Stuff foam sealants (like Gaps & Cracks and Window & Door) are generally considered closed-cell. Closed cell injections would fare weather better than open cell. A gravel perimeter might do.

2. Bags of small gravel can be forced into voids, create a perimeter and topped with larger gravel resistant to erosion.

3. QUIKRETE fast setting conc.mix Red bag is another EZ solution @ only $7/50#

The better the anti-erosion perimeter & uphill grading, the longer lasting the solution.

Whatever works, please post results.

Get a block of wood and a prybar pry one side up an inch put sand or P gravel underneath it then go to the other side and do the same level then put something around the edge to create positive drainage to raise up the ground a little bit higher above the lower side of the concrete